As all structures in service require appropriate inspection and maintenance, they should be monitored for their integrity and health condition to prolong their life or to prevent catastrophic failure. Apparently, the structural health monitoring has become an important topic in recent years. Numerous methods have been employed to identify fault or damage of structures, where these methods may include conventional visual inspection and non-destructive techniques, such as ultrasonic and eddy current scanning, acoustic emission and X-ray inspection. These conventional methods require at least temporary removal of structures from service for inspection. Although still used for inspection of isolated locations, they are time-consuming and expensive.
With the advance of sensor technologies, new diagnostic techniques for in-situ structural integrity monitoring have been in significant progress. Typically, these new techniques utilize sensory systems of appropriate sensors and actuators built in host structures. However, these approaches have drawbacks and may not provide effective on-line methods to implement a reliable sensory network system and/or accurate monitoring methods that can diagnose, classify and forecast structural condition with the minimum intervention of human operators. For example, U.S. Pat. No. 5,814,729, issued to Wu et al., discloses a method that detects the changes of damping characteristics of vibrational waves in a laminated composite structure to locate delaminated regions in the structure. Piezoceramic devices are applied as actuators to generate the vibrational waves and fiber optic cables with different grating locations are used as sensors to catch the wave signals. A drawback of this system is that it cannot accommodate a large number of actuator arrays and, as a consequence, each of actuators and sensors must be placed individually. Since the damage detection is based on the changes of vibrational waves traveling along the line-of-sight paths between the actuators and sensors, this method fails to detect the damage located out of the paths and/or around the boundary of the structure.
Another approach for damage detection can be found in U.S. Pat. No. 5,184,516, issued to Blazic et al., that discloses a self-contained conformal circuit for structural health monitoring and assessment. This conformal circuit consists of a series of stacked layers and traces of strain sensors, where each sensor measures strain changes at its corresponding location to identify the defect of a conformal structure. The conformal circuit is a passive system, i.e., it does not have any actuator for generating signals. A similar passive sensory network system can be found in U.S. Pat. No. 6,399,939, issued to Mannur, J. et al. In Mannur '939 patent, a piezoceramic-fiber sensory system is disclosed having planner fibers embedded in a composite structure. A drawback of these passive methods is that they cannot monitor internal delamination and damages between the sensors. Moreover, these methods can detect the conditions of their host structures only in the local areas where the self-contained circuit and the piezoceramic-fiber are affixed.
One method for detecting damages in a structure is taught by U.S. Pat. No. 6,370,964 (Chang et al.). Chang et al. discloses a sensory network layer, called Stanford Multi-Actuator-Receiver Transduction (SMART) Layer. The SMART Layer® includes piezoceramic sensors/actuators equidistantly placed and cured with flexible dielectric films sandwiching the piezoceramic sensors/actuators (or, shortly, piezoceramics). The actuators generate acoustic waves and sensors receive/transform the acoustic waves into electric signals. To connect the piezoceramics to an electronic box, metallic clad wires are etched using the conventional flexible circuitry technique and laminated between the substrates. As a consequence, a considerable amount of the flexible substrate area is needed to cover the clad wire regions. In addition, the SMART Layer® needs to be cured with its host structure made of laminated composite layers. Due to the internal stress caused by a high temperature cycle during the curing process, the piezoceramics in the SMART Layer® can be micro-fractured. Also, the substrate of the SMART Layer® can be easily separated from the host structure. Moreover, it is very difficult to insert or attach the SMART Layer® to its host structure having a curved section and, as a consequence, a compressive load applied to the curved section can easily fold the clad wires. Fractured piezoceramics and the folded wires may be susceptible to electromagnetic interference noise and provide misleading electrical signals. In harsh environments, such as thermal stress, field shock and vibration, the SMART Layer® may not be a robust and unreliable tool for monitoring structural health. Furthermore, the replacement of damaged and/or defective actuators/sensors may be costly as the host structure needs to be dismantled.
Another method for detecting damages in a structure is taught by U.S. Pat. No. 6,396,262 (Light et al.). Light et al. discloses a magnetostrictive sensor for inspecting structural damages, where the sensor includes a ferromagnetic strip and a coil closely located to the strip. The major drawback of this system is that the system cannot be designed to accommodate an array of sensors and, consequently, cannot detect internal damages located between sensors.
Due to the mentioned drawbacks, the methodologies for analyzing data that are implemented in these conventional systems may have limitations in monitoring the host structures in an accurate and efficient manner. Thus, there is a need for new and efficient methodologies for analyzing and interpreting the data from the host systems to determine structural conditions and to prognosticate failures.